In the formation of a Field-Effect Transistor (FinFET), a semiconductor fin is formed first, followed by forming a gate stack on a portion of the semiconductor fin. The exposed end portions of the fin on opposite sides of the gate stack are removed to form recesses. Source and drain regions are then re-grown in the recesses through epitaxy.
In integrated circuits, transistors may need to have different threshold voltages. The threshold voltages of transistors may be adjusted by adjusting the channel doping concentrations in the transistors. In the FinFETs with narrow fins, however, the channel doping may fluctuate significantly. This is caused by thermal processes such as re-crystallization and activation processes. In the thermal processes, most of the channel dopants may be diffused away from fins, and the channel dopants that are left in the channel become an increasingly smaller portion with the increasingly narrowing of fins. As a result, heavier doses of impurities are needed.
Increasing the implantation doses, however, results in high threshold voltage variations. The out-diffusion of the impurity from a narrow fin is affected by various factors, and has high variations. From wafer to wafer, and even from FinFET to FinFET on the same wafer, the out-diffusion may vary significantly. This results in the channel doping concentration of the FinFETs to vary also. The threshold voltages of the resulting FinFETs thus have a high variation.